The purpose of this research project is to demonstrate the usefulness of the single-crystal neutron diffraction method (which can clearly distinguish hydrogen from deuterium) for determining the absolute configuration of molecules having chiral methylene groups (i.e., molecules of the type *CHDRR'). The key idea in our proposal is to incorporate into the crystalline sample a chiral organic fragment of known absolute configuration (such as a naturally-occurring alkaloid) as a reference from which the absolute configuraton of the chiral methylene group can be obtained. With this method, we hope to provide final conclusive proof of the absolute stereochemistry of several important enzymatic reactions that produce *CHDRR'-type molecules. Specific examples of systems that we intend to study include the acetaldehyde/ethanol, fumarate/malate and malate/succinate transformations. In each case, neutron diffraction analysis will be carried out on salts of the stereospecifically-labelled anions with optically-active cations. Future plans will involve extending this work to systems of unresolved stereochemistry, such as the formation of 1-aminocyclopropane-1-carboxylic acid from S-adenosylmethionine.